RTP SMPTE ST 336: RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) ST 336 Encoded Data
How to carry SMPTE KLV metadata, the labelled data that travels with video, inside RTP packets.
Downs, J., Ed. and J. Arbeiter, Ed., "RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) ST 336 Encoded Data", RFC 6597, DOI 10.17487/RFC6597, April 2012, <https://www.rfc-editor.org/info/rfc6597>.
@misc{rfc6597,
series = {Request for Comments},
number = 6597,
howpublished = {RFC 6597},
publisher = {RFC Editor},
doi = {10.17487/RFC6597},
url = {https://www.rfc-editor.org/info/rfc6597},
author = {J. Downs and J. Arbeiter},
title = {{RTP Payload Format for Society of Motion Picture and Television Engineers (SMPTE) ST 336 Encoded Data}},
pagetotal = 13,
year = 2012,
month = apr,
} RFC text
Contents
Internet Engineering Task Force (IETF) J. Downs, Ed.
Request for Comments: 6597 PAR Government Systems Corp.
Category: Standards Track J. Arbeiter, Ed.
ISSN: 2070-1721 April 2012
RTP Payload Format for
Society of Motion Picture and Television Engineers (SMPTE)
ST 336 Encoded Data
Abstract
This document specifies the payload format for packetization of KLV
(Key-Length-Value) Encoded Data, as defined by the Society of Motion
Picture and Television Engineers (SMPTE) in SMPTE ST 336, into the
Real-time Transport Protocol (RTP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6597.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................2
2. Conventions, Definitions, and Acronyms ..........................3
3. Media Format Background .........................................3
4. Payload Format ..................................................4
4.1. RTP Header Usage ...........................................5
4.2. Payload Data ...............................................5
4.2.1. The KLVunit .........................................5
4.2.2. KLVunit Mapping to RTP Packet Payload ...............6
4.3. Implementation Considerations ..............................6
4.3.1. Loss of Data ........................................6
4.3.1.1. Damaged KLVunits ...........................7
4.3.1.2. Treatment of Damaged KLVunits ..............9
5. Congestion Control ..............................................9
6. Payload Format Parameters .......................................9
6.1. Media Type Definition ......................................9
6.2. Mapping to SDP ............................................10
6.2.1. Offer/Answer Model and Declarative Considerations ..10
7. IANA Considerations ............................................11
8. Security Considerations ........................................11
9. References .....................................................12
9.1. Normative References ......................................12
9.2. Informative References ....................................12
1. Introduction
This document specifies the payload format for packetization of KLV
(Key-Length-Value) Encoded Data, as defined by the Society of Motion
Picture and Television Engineers (SMPTE) in [SMPTE-ST336], into the
Real-time Transport Protocol (RTP) [RFC3550].
The payload format is defined in such a way that arbitrary KLV data
can be carried. No restrictions are placed on which KLV data keys
can be used.
A brief description of SMPTE ST 336, "Data Encoding Protocol Using
Key-Length-Value", is given. The payload format itself, including
use of the RTP header fields, is specified in Section 4. The media
type and IANA considerations are also described. This document
concludes with security considerations relevant to this payload
format.
2. Conventions, Definitions, and Acronyms
The term "Universal Label Key" is used in this document to refer to a
fixed-length, 16-byte SMPTE-administered Universal Label (see
[SMPTE-ST298]) that is used as an identifying key in a KLV item.
The term "KLV item" is used in this document to refer to one single
Universal Label Key, length, and value triplet encoded as described
in [SMPTE-ST336].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Media Format Background
[SMPTE-ST336], "Data Encoding Protocol Using Key-Length-Value",
defines a byte-level data encoding protocol for representing data
items and data groups. This encoding protocol definition is
independent of the application or transportation method used.
SMPTE ST 336 data encoding can be applied to a wide variety of binary
data. This encoding has been used to provide diverse and rich
metadata sets that describe or enhance associated video
presentations. Use of SMPTE ST 336 encoded metadata in conjunction
with video has enabled improvements in multimedia presentations,
content management and distribution, archival and retrieval, and
production workflow.
The SMPTE ST 336 standard defines a KLV triplet as a data interchange
protocol for data items or data groups where the Key identifies the
data, the Length specifies the length of the data, and the Value is
the data itself. The KLV protocol provides a common interchange
point for all compliant applications irrespective of the method of
implementation or transport.
The Key of a KLV triplet (a Universal Label Key) is coded using a
fixed-length 16-byte SMPTE-administered Universal Label.
[SMPTE-ST298] further details the structure of 16-byte SMPTE-
administered Universal Labels. Universal Label Keys are maintained
in registries published by SMPTE (see, for example, [SMPTE-ST335] and
[SMPTE-RP210]).
The standard also provides methods for combining associated KLV
triplets in data sets where the set of KLV triplets is itself coded
with the KLV data coding protocol. Such sets can be coded in either
full form (Universal Sets) or one of four increasingly bit-efficient
forms (Global Sets, Local Sets, Variable Length Packs, and Defined
Length Packs). The standard provides a definition of each of these
data constructs.
Additionally, the standard defines the use of KLV coding to provide a
means to carry information that is registered with a non-SMPTE
external agency.
4. Payload Format
The main goal of the payload format design for SMPTE ST 336 data is
to provide carriage of SMPTE ST 336 data over RTP in a simple, yet
robust manner. All forms of SMPTE ST 336 data can be carried by the
payload format. The payload format maintains simplicity by using
only the standard RTP headers and not defining any payload headers.
SMPTE ST 336 KLV data is broken into KLVunits. A KLVunit is simply a
logical grouping of otherwise unframed KLV data, grouped based on
source data timing (see Section 4.2.1). Each KLVunit is then placed
into one or more RTP packet payloads. The RTP header marker bit is
used to assist receivers in locating the boundaries of KLVunits.
4.1. RTP Header Usage
This payload format uses the RTP packet header fields as described in
the table below:
+-----------+-------------------------------------------------------+
| Field | Usage |
+-----------+-------------------------------------------------------+
| Timestamp | The RTP Timestamp encodes the instant along a |
| | presentation timeline that the entire KLVunit encoded |
| | in the packet payload is to be presented. When one |
| | KLVunit is placed in multiple RTP packets, the RTP |
| | timestamp of all packets comprising that KLVunit MUST |
| | be the same. The timestamp clock frequency is |
| | defined as a parameter to the payload format |
| | (Section 6). |
| | |
| M-bit | The RTP header marker bit (M) is used to demarcate |
| | KLVunits. Senders MUST set the marker bit to '1' for |
| | any RTP packet that contains the final byte of a |
| | KLVunit. For all other packets, senders MUST set the |
| | RTP header marker bit to '0'. This allows receivers |
| | to pass a KLVunit for parsing/decoding immediately |
| | upon receipt of the last RTP packet comprising the |
| | KLVunit. Without this, a receiver would need to wait |
| | for the next RTP packet with a different timestamp to |
| | arrive, thus signaling the end of one KLVunit and the |
| | start of another. |
+-----------+-------------------------------------------------------+
The remaining RTP header fields are used as specified in [RFC3550].
4.2. Payload Data
4.2.1. The KLVunit
A KLVunit is a logical collection of all KLV items that are to be
presented at a specific time. A KLVunit is comprised of one or more
KLV items. Compound items (sets, packs) are allowed as per
[SMPTE-ST336], but the contents of a compound item MUST NOT be split
across two KLVunits. Multiple KLV items in a KLVunit occur one after
another with no padding or stuffing between items.
4.2.2. KLVunit Mapping to RTP Packet Payload
An RTP packet payload SHALL contain one, and only one, KLVunit or a
fragment thereof. KLVunits small enough to fit into a single RTP
packet (RTP packet size is up to the implementation but should
consider underlying transport/network factors such as MTU
limitations) are placed directly into the payload of the RTP packet,
with the first byte of the KLVunit (which is the first byte of a KLV
Universal Label Key) being the first byte of the RTP packet payload.
KLVunits too large to fit into a single RTP packet payload MAY span
multiple RTP packet payloads. When this is done, the KLVunit data
MUST be sent in sequential byte order, such that when all RTP packets
comprising the KLVunit are arranged in sequence number order,
concatenating the payload data together exactly reproduces the
original KLVunit.
Additionally, when a KLVunit is fragmented across multiple RTP
packets, all RTP packets transporting the fragments of a KLVunit MUST
have the same timestamp.
KLVunits are bounded with changes in RTP packet timestamps. The
marker (M) bit in the RTP packet headers marks the last RTP packet
comprising a KLVunit (see Section 4.1).
4.3. Implementation Considerations
4.3.1. Loss of Data
RTP is generally deployed in network environments where packet loss
might occur. RTP header fields enable detection of lost packets, as
described in [RFC3550]. When transmitting payload data described by
this payload format, packet loss can cause the loss of whole KLVunits
or portions thereof.
4.3.1.1. Damaged KLVunits
A damaged KLVunit is any KLVunit that was carried in one or more RTP
packets that have been lost. When a lost packet is detected (through
use of the sequence number header field), the receiver
o MUST consider the KLVunit partially received before a lost packet
as damaged. This damaged KLVunit includes all packets prior to
the lost one (in sequence number order) back to, but not
including, the most recent packet in which the M-bit in the RTP
header was set to '1'.
o MUST consider the first KLVunit received after a lost packet as
damaged. This damaged KLVunit includes the first packet after the
lost one (in sequence number order) and, if the first packet has
its M-bit in the RTP header set to '0', all subsequent packets up
to and including the next one with the M-bit in the RTP header set
to '1'.
The above applies, regardless of the M-bit value in the RTP header of
the lost packet itself. This enables very basic receivers to look
solely at the M-bit to determine the outer boundaries of damaged
KLVunits. For example, when a packet with the M-bit set to '1' is
lost, the KLVunit that the lost packet would have terminated is
considered damaged, as is the KLVunit comprised of packets received
subsequent to the lost packet (up to and including the next received
packet with the M-bit set to '1').
The example below illustrates how a receiver would handle a lost
packet in another possible packet sequence:
+---------+-------------+ +--------------+
| RTP Hdr | Data | | |
+---------+-------------+ +--------------+
.... | ts = 30 | KLV KLV ... | | | >---+
| M = 1 | | | | |
| seq = 5 | ... KLV KLV | | | |
+---------+-------------+ +--------------+ |
Last RTP pkt for time 30 Lost RTP Pkt |
(seq = 6) |
|
+--------------------------------------------------------+
|
| +---------+-------------+ +---------+-------------+
| | RTP Hdr | Data | | RTP Hdr | Data |
| +---------+-------------+ +---------+-------------+
+--> | ts = 45 | KLV KLV ... | | ts = 45 | ... KLV ... | >---+
| M = 0 | | | M = 1 | | |
| seq = 7 | ... KLV ... | | seq = 8 | ... KLV KLV | |
+---------+-------------+ +---------+-------------+ |
RTP pkt for time 45 Last RTP pkt for time 45 |
KLVunit carried in these two packets is "damaged" |
|
+----------------------------------------------------------------+
|
| +---------+-------------+
| | RTP Hdr | Data |
| +---------+-------------+
+--> | ts = 55 | KLV KLV ... | ....
| M = 1 | |
| seq = 9 | ... KLV ... |
+---------+-------------+
Last and only RTP pkt
for time 55
In this example, the packets with sequence numbers 7 and 8 contain
portions of a KLVunit with a timestamp of 45. This KLVunit is
considered "damaged" due to the missing RTP packet with sequence
number 6, which might have been part of this KLVunit. The KLVunit
for timestamp 30 (ended in packet with sequence number 5) is
unaffected by the missing packet. The KLVunit for timestamp 55,
carried in the packet with sequence number 9, is also unaffected by
the missing packet and is considered complete and intact.
4.3.1.2. Treatment of Damaged KLVunits
SMPTE ST 336 KLV data streams are built in such a way that it is
possible to partially recover from errors or missing data in a
stream. Exact specifics of how damaged KLVunits are handled are left
to each implementation, as different implementations can have
differing capabilities and robustness in their downstream KLV payload
processing. Because some implementations can be particularly limited
in their capacity to handle damaged KLVunits, receivers MAY drop
damaged KLVunits entirely.
5. Congestion Control
The general congestion control considerations for transporting RTP
data apply; see RTP [RFC3550] and any applicable RTP profile, like
AVP [RFC3551].
Further, SMPTE ST 336 data can be encoded in different schemes that
reduce the overhead associated with individual data items within the
overall stream. SMPTE ST 336 grouping constructs, such as local sets
and data packs, provide a mechanism to reduce bandwidth requirements.
6. Payload Format Parameters
This RTP payload format is identified using the application/smpte336m
media type, which is registered in accordance with [RFC4855], and
using the template of [RFC4288].
6.1. Media Type Definition
Type name: application
Subtype name: smpte336m
Required parameters:
rate: RTP timestamp clock rate. Typically chosen based on
sampling rate of metadata being transmitted, but other rates
can be specified.
Optional parameters: None
Encoding considerations: This media type is framed and binary; see
Section 4.8 of [RFC4288].
Security considerations: See Section 8 of RFC 6597.
Interoperability considerations: Data items in smpte336m can be very
diverse. Receivers might only be capable of interpreting a subset
of the possible data items; unrecognized items are skipped.
Agreement on data items to be used out of band, via application
profile or similar, is typical.
Published specification: RFC 6597
Applications that use this media type: Streaming of metadata
associated with simultaneously streamed video and transmission of
[SMPTE-ST336]-based media formats (e.g., Material Exchange Format
(MXF) [SMPTE-ST377]).
Additional Information: none
Person & email address to contact for further information: J. Downs
<jeff_downs@partech.com>; IETF Payload Working Group
<payload@ietf.org>
Intended usage: COMMON
Restrictions on usage: This media type depends on RTP framing, and
hence is only defined for transfer via RTP ([RFC3550]). Transport
within other framing protocols is not defined at this time.
Author:
J. Downs <jeff_downs@partech.com>
J. Arbeiter <jimsgti@gmail.com>
Change controller: IETF Payload working group delegated from the
IESG.
6.2. Mapping to SDP
The mapping of the above defined payload format media type and its
parameters SHALL be done according to Section 3 of [RFC4855].
6.2.1. Offer/Answer Model and Declarative Considerations
This payload format has no configuration or optional format
parameters. Thus, when offering SMPTE ST 336 Encoded Data over RTP
using the Session Description Protocol (SDP) in an Offer/Answer model
[RFC3264] or in a declarative manner (e.g., SDP in the Real-Time
Streaming Protocol (RTSP) [RFC2326] or the Session Announcement
Protocol (SAP) [RFC2974]), there are no specific considerations.
7. IANA Considerations
IANA has registered application/smpte336m as specified in
Section 6.1. The media type has been added to the IANA registry for
"RTP Payload Format media types"
(http://www.iana.org/assignments/rtp-parameters).
8. Security Considerations
RTP packets using the payload format defined in this specification
are subject to the security considerations discussed in the RTP
specification [RFC3550], and in any applicable RTP profile. The main
security considerations for the RTP packet carrying the RTP payload
format defined within this memo are confidentiality, integrity, and
source authenticity. Confidentiality is achieved by encryption of
the RTP payload. Integrity of the RTP packets is achieved through a
suitable cryptographic integrity protection mechanism. Cryptographic
systems may also allow the authentication of the source of the
payload. A suitable security mechanism for this RTP payload format
should provide confidentiality, integrity protection, and at least
source authentication capable of determining whether or not an RTP
packet is from a member of the RTP session.
Note that the appropriate mechanism to provide security to RTP and
payloads following this memo may vary. It is dependent on the
application, the transport, and the signaling protocol employed.
Therefore, a single mechanism is not sufficient, although if suitable
the usage of the Secure Real-time Transport Protocol (SRTP) [RFC3711]
is recommended. Other mechanisms that may be used are IPsec
[RFC4301] and Transport Layer Security (TLS) [RFC5246] (RTP over
TCP), but other alternatives may exist as well.
This RTP payload format presents the possibility for significant
non-uniformity in the receiver-side computational complexity during
processing of SMPTE ST 336 payload data. Because the length of SMPTE
ST 336 encoded data items is essentially unbounded, receivers must
take care when allocating resources used in processing. It is easy
to construct pathological data that would cause a naive decoder to
allocate large amounts of resources, resulting in denial-of-service
threats. Receivers SHOULD place limits on resource allocation that
are within the bounds set forth by any application profile in use.
This RTP payload format does not contain any inherently active
content. However, individual SMPTE ST 336 KLV items could be defined
to convey active content in a particular application. Therefore,
receivers capable of decoding and interpreting such data items should
use appropriate caution and security practices. In particular,
accepting active content from streams that lack authenticity or
integrity protection mechanisms places a receiver at risk of attacks
using spoofed packets. Receivers not capable of decoding such data
items are not at risk; unknown data items are skipped over and
discarded according to SMPTE ST 336 processing rules.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio
and Video Conferences with Minimal Control", STD 65,
RFC 3551, July 2003.
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications
and Registration Procedures", BCP 13, RFC 4288,
December 2005.
[RFC4855] Casner, S., "Media Type Registration of RTP Payload
Formats", RFC 4855, February 2007.
9.2. Informative References
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session
Announcement Protocol", RFC 2974, October 2000.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer
Model with Session Description Protocol (SDP)",
RFC 3264, June 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and
K. Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
[SMPTE-RP210] Society of Motion Picture and Television Engineers,
"SMPTE RP 210v12:2010 Data Element Dictionary", 2010,
<http://www.smpte-ra.org/mdd/>.
[SMPTE-ST298] Society of Motion Picture and Television Engineers,
"SMPTE ST 298:2009 Universal Labels for Unique
Identification of Digital Data", 2009,
<http://www.smpte.org>.
[SMPTE-ST335] Society of Motion Picture and Television Engineers,
"SMPTE ST 335:2012 Metadata Element Dictionary
Structure", 2012, <http://www.smpte.org>.
[SMPTE-ST336] Society of Motion Picture and Television Engineers,
"SMPTE ST 336:2007 Data Encoding Protocol Using Key-
Length-Value", 2007, <http://www.smpte.org>.
[SMPTE-ST377] Society of Motion Picture and Television Engineers,
"SMPTE ST 377-1:2011 Material Exchange Format (MXF) -
File Format Specification", 2011,
<http://www.smpte.org>.
Authors' Addresses
J. Downs (editor)
PAR Government Systems Corp.
US
EMail: jeff_downs@partech.com
J. Arbeiter (editor)
US
EMail: jimsgti@gmail.com
AI ELI5
On this page
§1Introduction and Scope
RFC 6597 specifies how to packetize KLV (Key-Length-Value) encoded data, as defined by the Society of Motion Picture and Television Engineers in SMPTE ST 336, into the Real-time Transport Protocol (RFC 3550).
The format is built so that arbitrary KLV data can be carried. No restrictions are placed on which KLV data keys can be used.
It is a set of rules for putting small labelled notes about a video into network packets. Any kind of note is allowed.
§3KLV and SMPTE ST 336
SMPTE ST 336 defines a byte-level data encoding for data items and data groups, independent of the application or transport. A KLV triplet has a Key that identifies the data, a Length that gives its size, and a Value that is the data itself.
The Key is a fixed-length 16-byte SMPTE-administered Universal Label. Its structure is described in SMPTE ST 298, and keys are kept in registries published by SMPTE. The RFC says this encoding has been used for metadata that describes or enhances video presentations.
KLV triplets can be combined into sets that are themselves KLV coded: the full form (Universal Sets) or one of four more compact forms (Global Sets, Local Sets, Variable Length Packs and Defined Length Packs). KLV can also carry information registered with a non-SMPTE external agency.
Each note is a labelled box: a name tag, a size, and the contents. Boxes can be packed inside bigger boxes to save space.
§4KLVunits and the RTP Header
The design goal is simple but robust carriage of all forms of ST 336 data. It stays simple by using only the standard RTP headers and defining no payload headers.
The data is split into KLVunits: the logical collection of all KLV items to be presented at a specific time. A unit is one or more KLV items, one after another with no padding or stuffing. Compound items such as sets and packs are allowed, but their contents MUST NOT be split across two KLVunits.
Two RTP header fields get special use. The timestamp gives the instant on the presentation timeline at which the whole KLVunit is presented, and its clock frequency is a payload format parameter. The marker bit (M) is set to 1 on any packet that contains the final byte of a KLVunit and to 0 on all other packets. All other header fields are used as in RFC 3550.
All notes meant for the same moment go in one bundle. Every packet of a bundle gets the same time stamp, and the last packet gets a flag saying it is the end.
§5Mapping a KLVunit to Packets
An RTP payload SHALL contain one, and only one, KLVunit or a fragment of one. Packet size is up to the implementation, which should consider limits such as the MTU. A small unit goes directly into one payload, so the first payload byte is the first byte of a Universal Label Key.
A unit too large for one packet MAY span several. Its data MUST be sent in sequential byte order, so that putting the packets in sequence number order and joining their payloads exactly reproduces the original unit. All of those packets MUST carry the same timestamp.
Because the marker bit flags the last packet, a receiver can hand a unit to the decoder as soon as that packet arrives. Without it, the receiver would have to wait for a packet with a different timestamp to learn that the unit had ended.
If a bundle is too big for one envelope, you split it across several in order, with the same time stamp on each. The last envelope is marked so the reader knows to stop.
§6Lost Packets and Damaged KLVunits
Packet loss can lose whole KLVunits or parts of them. Lost packets are detected through the RTP sequence number. A receiver MUST treat two groups as damaged KLVunits: the unit partially received before the lost packet (back to, but not including, the latest packet with M set to 1), and the first unit received after the lost packet (the first packet after the loss and, if its M bit is 0, all following packets up to and including the next with M set to 1).
This applies whatever the lost packet's own M bit was, so very basic receivers can find the outer bounds of damaged units by looking only at the M bit.
What to do with a damaged unit is left to each implementation. KLV streams allow partial recovery, but because some implementations are limited, receivers MAY drop damaged units entirely.
If an envelope goes missing, the bundle it belonged to and the bundle right after it might be incomplete. The reader may throw those bundles away or try to save what it can.
- seq 5, ts 30, M=1Last packet of the unit for time 30; unaffected.
- seq 6 (lost)Never arrives.
- seq 7, ts 45, M=0First packet after the loss; its unit is damaged.
- seq 8, ts 45, M=1Ends the damaged unit.
- seq 9, ts 55, M=1Single-packet unit; complete and intact.
§7Media Type and SDP
The payload format uses the media type application/smpte336m. Its one required parameter is rate, the RTP timestamp clock rate, typically chosen from the sampling rate of the metadata. There are no optional parameters.
The type is framed and binary and is defined only for transfer via RTP. Receivers might understand only some data items, and unrecognized items are skipped, so which items are used is typically agreed out of band. SDP mapping follows Section 3 of RFC 4855. The format has no configuration or optional format parameters, so Offer/Answer and declarative use raise no special considerations. IANA registered the media type.
The format has a name tag, and the only thing you must say when using it is how fast its clock ticks.
§8Security and Congestion
The usual RTP concerns apply: confidentiality, integrity and source authenticity. No single mechanism fits every case, but SRTP is recommended if suitable; IPsec and TLS are named as other options.
Because KLV item lengths are essentially unbounded, it is easy to build data that makes a naive decoder allocate large resources, a denial-of-service threat. Receivers SHOULD limit resource allocation within the bounds of any application profile in use. The format has no inherently active content, but individual KLV items could be defined to carry some. Accepting such content from streams without authenticity or integrity protection exposes receivers to spoofed-packet attacks. Receivers that cannot decode such items are not at risk, since unknown items are skipped.
For congestion control, the general RTP rules apply. ST 336 grouping constructs such as local sets and data packs can reduce bandwidth.
Someone could send a fake note claiming to be enormous to make the reader run out of room. So readers set limits, and it is wise to lock the packets so only trusted senders get through.